Packaged precooked meat and gravy combinations are commonly available to the public in vacuum sealed airtight packets, such as those made of pliable plastic films. The packets are termed airtight to the extent that the films are relatively impermeable to air, so that the components sealed within them at the time of preparation remain largely anaerobic. Such products must be prepared free of pathogenic organisms, especially toxin-producing anaerobes. Pathogenic organisms that may contaminate packaged meats include, by way of nonlimiting example, Clostridium botulinum, C. perfringens, (Lucke et al., in "Ecology and Control Foods" (A.H.W. Hauschild and K.L. Dodds, eds.) Marcel Dekker, N.Y., 1993, pp. 177-207; Smart et al., J. Appl. Bacteriol. 46, 377-383 (1979); Roberts et al., J. Fd. Technol. 14, 211-226 (1979); Tompkin, Food Technology 34, 229-236, and 257 (1980); Bryan et al., Amer. Public Health 61, 1869-1885 (1971); Microbial Ecology of Food Commodities--Microorganisms in Foods 6: Blackie Academic and Professional, 1998, p. 115), Listeria monocytogenes, Escherichia coli, Bacillus cereus, Enterococcus faecalis, and similar microorganisms. Among these, spore-forming, toxin-producing microorganisms are of particular concern, because any spores produced by viable cells may survive and grow to produce toxins subsequent to manufacturing or domestic heating steps. Such microorganisms include species of the genus Clostridium.
In U.S. Pat. No. 4,888,191 and 5,017,391 Anders et al. disclose compositions and methods related to the use of lactate salts to delay C. botulinum growth in a foodstuff such as fish or poultry. The foods are heated to a temperature sufficient to cook the meat but not to sterilize the product. Anders et al. suggest that lactate may be used alone, or in combination with other agents such as sodium nitrite. These patents fail to discuss nisin or its properties.
Maas et al. (Appl. Envir. Microbiol. 55, 2226-2229 (1989)) report that lactate, when incorporated into a turkey meat vacuum-packed composition, delays the generation of botulinum toxin in a manner directly dependent on the concentration of lactate introduced into the composition. Maas et al. do not mention nisin.
Nisin is a peptide-like antibacterial substance produced by microorganisms such as Lactococcus lactis subsp. lactis (formerly known as Streptococcus lactis). Its structure is illustrated in U.S. Pat. No. 5,527,505 to Yamauchi et al. The highest activity preparations of nisin contain about 40 million IU per gram. A commercial preparation, NISAPLIN.TM., containing about 1 million IU per gram is available from Aplin & Barrett Ltd., Trowbridge, England. Nisin has no known toxic effects in humans. It is widely used in a variety of prepared dairy foods. Experimental use in preserving other foods has also been reported. Details on these applications are provided below.
A number of efforts have been reported since 1975 directed to reducing uncoupled acid production in dairy fermentations by controlling the post-fermentation acidification of yogurt. In some of these studies, a nisin producing culture was introduced in an attempt to inhibit these effects. Kalra et al. (Indian Journal of Dairy Science 28: 71-72 (1975)) incorporated the nisin producing culture Streptococcus lactis (now known as L. lactis subsp. lactis) along with the yogurt culture before fermentation. Others introduced nisin in milk prior to fermentation (Bayoumi, Chem. mikrobiol. technol. lebensm. 13:65-69 (1991)) or following fermentation (Gupta et al., Cultured Dairy Products Journal 23: 17-18 (1988); Gupta et al., Cultured Dairy Products Journal 23: 9-10 (1989)). In all cases, the rate of post-fermentation acidification was only partially inhibited by these treatments and the yogurt continued to become more acidic throughout its shelf life.
In U.S. Pat. No. 5,527,505, by Yamauchi et al., yogurt was produced from raw milk by incorporating a nisin-producing strain, Lactococcus lactis subsp. lactis, along with the traditional yogurt culture consisting of Streptococcus salivarius subsp. thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). Yamauchi et al. teach that the lactococci are needed to secrete the nisin, whose effect is to retard the activity of ST and LB. The resulting yogurt therefore contains the lactococci used to produce the nisin. Nonetheless, the acidity of yogurt containing the nisin-producing bacteria increased by 64% to 96% in 14 days, in various experiments inoculated with differing amounts of L. lactis subsp. lactis, compared to the initial acidity at the completion of fermentation. Other studies (Hogarty et al., J. Fd. Prot. 45:1208-1211 (1982); Sadovski et al., XX International Dairy Congress, Vol. E: 542-5-44 (1978)) also noted acid production and development of bitterness at low temperature by some mesophilic starter lactococci in dairy products.
In U.S. Pat. No. 5,015,487 to Collison et al., the use of nisin, as a representative of the class of lanthionine bacteriocins, to control undesirable microorganisms in heat processed meats is disclosed. In tests involving dipping frankfurters in nisin solutions, the growth of L. monocytogenes was effectively inhibited upon storage at 4.degree. C.
Chung et al. (Appl. Envir. Microbiol. 55, 1329-1333 (1989)) report that nisin has an inhibitory effect on gram-positive bacteria, such as L. monocytogenes, Staphylococcus aureus and Streptococcus lactis, but has no such effect on gram-negative bacteria such as Serratia marcescens, Salmonella typhimurium and Pseudomonas aeruginosa when these microorganisms are attached to meat.
Nisin has been added to cheeses to inhibit toxin production by Clostridium botulinum (U.S. Pat. No. 4,584,199 to Taylor). U.S. Pat. No. 4,597,972 to Taylor discloses a detailed example in which chicken frankfurter components are shown to require the presence of both added nitrite and added nisin in order to prevent or delay botulinum toxin production when challenged with C. botulinum.
Nisaplin.TM. has been found to preserve salad dressings from microbiological contamination, such as challenge by Lactobacillus brevis subsp. lindneri, for an extended shelf life period (Muriana et al., J. Food Protection 58:1109-1113 (1995)).
There remains a need for compositions and procedures related to prepared meat products that inhibit the growth of pathogenic microorganisms, and the production of toxins by them, using natural or innocuous ingredients. In particular, there is a need for such compositions and methods that avoid the introduction of nitrite, the use of which has been the subject of extended controversy. The present invention addresses this need.